What’s it mean that an Irishman and a Scotsman— yourself and Ewan McGregor — are in a movie that does such a job on an English prime minister?

Oh, that’s the Polanski sense of humor, an Irishman playing the ex-prime minister. That didn’t get beyond me. But I’m not sure why he wanted me, and I didn’t ask. I thought, let sleeping dogs lie. Let’s just have fun. Let’s just play. He likes actors from the British Isles.

Roman with friend Catherine Deneuve.

What made working with Polanski better, or different, than with other directors?

Well, for one thing we shot my last scene in the movie first. We rehearsed in Roman’s trailer and he said, “OK, let’s shoot.” And we did anything but shoot. He fussed with the props, he fussed with the computers, he fussed with the guns, he fussed with my security men and then, right before lunch, he said, “OK, Pierce, after lunch, 27 lens . . .” and that’s a big lens, right in your face. And then we shot the scene.

And why did you want to work with him?

The man comes out of such a turbulent past and such a history of cinema and tragedy. I’d never met him, but the day I went to have lunch with him in Paris, I already knew him — what he sounded like, what he looked like, his life. It was a great invitation, a wonderful time, a magnificent director. He’s a unique character and wonderful filmmaker, and this character that I play was a great way to step out and play a political thriller.